|By Nicole Nee and Imogen Braddick

|By Nicole Nee and Imogen Braddick

|By Nicole Nee and Imogen Braddick

SINGAPORE (Reuters) - Singapore confirmed 15 new cases of locally-transmitted Zika infections on Monday, taking the tally to 56 as authorities step up efforts to contain the outbreak.

All the cases were in or near the Aljunied area in the southeast of the city-state, and most were foreign workers from a nearby construction site owned by GuocoLand, where testing for the virus is now complete, the health ministry said in a statement.

The National Environment Agency (NEA) has inspected foreign workers' dormitories as well as thousands of other premises in the area and in six other parts of the island.

Singapore, a major regional financial center and busy transit hub, which maintains a constant vigil against the mosquito-borne dengue virus, reported its first Zika case in May, imported by a middle-aged man who had been to Brazil.

On Saturday, the health ministry confirmed a first locally-transmitted case, with the tally jumping to 41 a day later amid warnings of more likely positive cases.

One of the cases discovered on Sunday involved a Singaporean man who works at the GuocoLand site but who lives outside the Aljunied area. The NEA inspected more than 900 premises around his home on Monday.

"The NEA went to every house, checked the kitchen and bathroom. They gave us a small bottle of insect repellent, but we just went to get a big bottle today," said 36-year-old local resident Tommy Fan. "I'm a little worried since my wife is trying to conceive. Zika seems to ... (spread) faster than dengue."

One local pharmacy worker said she ran out of mosquito repellent, and was concerned about delays in getting fresh supplies.

The Zika virus, carried by mosquitoes, was detected in Brazil last year and has since spread across the Americas.

It poses a risk to pregnant women because it can cause severe birth defects. It has been linked in Brazil to more than 1,600 cases of microcephaly, where babies are born with abnormally small heads and brains.

The World Health Organization said on Sunday it did not know "which lineage of Zika is circulating" in Singapore or "what the level of population immunity is to this lineage in Asia."

Singapore's health ministry said around three dozen mosquito breeding habitats had been destroyed so far in the main Aljunied cluster.

"I feel afraid," said Ng Kai Yee, an 18-year-old female student who lives near the outbreak area. "I heard quite a lot about how harmful Zika virus is to girls, especially pregnant women."

Authorities have urged those living and working in the risk areas, especially pregnant women, to monitor their health and seek medical attention if they are unwell.

Of the 41 people confirmed on Sunday to have been infected, 34 have fully recovered. Only the first case reported was a woman.

Regional health experts said the Zika virus is likely to be significantly under-reported across tropical Southeast Asia as local health authorities fail to conduct adequate screening.

Malaysia and Indonesia have stepped up protective measures following the Singapore outbreak, intensifying checks on people arriving from Singapore and introducing thermal scanners at airports and border checkpoints.